Asymmetric Digital Subscriber Line (ADSL) is defined, and pertinent standards information thereto is provided, in the Standards Document T1E1.4/98-007R1 entitled "Standards Project for Interfaces Relating to Carrier to Customer Connection of Asymmetrical Digital Subscriber Line (ADSL) Equipment", Sept. 26, 1997, and subsequent revisions edited by Bingham and Van der Putten. More particularly, Asymmetric Digital Subscriber Line (ADSL) has been defined as consisting of a pair of Discrete Multitone (DMT) modems, one at each end of a twisted pair subscriber loop, with band splitters at each end to provide isolation between a high frequency spectrum (above 24 KHz) used for data transmission and lower frequencies (0-4 KHz) used for Plain Old Telephone Service (POTS) or Integrated Services Digital Network (ISDN) used in some applications. These modems use DMT technology to provide high data bandwidth and the ability to adapt the data rate at startup to match the transmission and noise characteristics of each individual subscriber loop. Each 4.3125 KHz band of the frequency spectrum (referred to as a Tone Bin), with center frequencies from 25.875 KHz (Tone Bin #6) to 1.04 MHz, is populated with a signal which may be considered to be a tone whose amplitude and phase are modulated to encode data bits. Tone Bins below 24 KHz (e.g., Tone Bins #1-#5) are not used for data transmissions.
Proposals have been made by various companies to define an alternate spectral assignment for Asymmetric Digital Subscriber Line (ADSL) Discrete Multitone (DMT) modems that allow for the use of low frequencies (below 24 KHz) for additional data throughput. In existing classical ADSL systems, the lower frequencies including the 0-4 KHz voice band and approximately 20 KHz of guard band thereabove, are reserved for Plain Old Telephone Service (POTS) telephony voice and signaling transmissions in some applications. In these classical ADSL systems, the ADSL data transmissions are transmitted in frequency bands above 24 KHz (Tone Bin #6 and above) with the different transmissions separated by filters in central office and Customer Premises Equipment (CPE) splitters.
Recent interest in "splitterless" ADSL has generated an activity in trying to provide a dual modality for transmissions wherein a subscriber's line could be used either in the classical ADSL mode as described above, or in a data only mode. In the data only mode, the lower frequencies (0-24 KHz) would be used to carry additional DMT "tones" (Tone Bins #1-#5) modulated with additional data. Since the lower frequencies are transmitted preferentially by most telephone loops, these lower frequencies can add substantial throughput capability to the subscriber's line.
The recent focus has been on eliminating the band splitter, at least at the subscriber premises. This necessitates some reduction in data throughput, since data carrier amplitudes must be reduced to avoid interference to POTS voice generated by nonlinearities. Since the POTS phone impedance loads the subscriber loop, it reduces input amplitude and available spectral bandwidth to the data receivers. Alternatively, POTS filters may be installed on each POTS telephone to prevent the ADSL signal from reaching, or being adversely affected by, the telephone, whether on-hook or off-hook. Unfortunately, while these POTS filters will reduce, or substantially eliminate, the POTS signal interference and the need to reduce the ADSL signal amplitude, the filters may render one or two of the tones below Tone #6 unusable. This reduction in data bandwidth is viewed as an acceptable tradeoff to reduce the complexity of installation of the service by eliminating equipment rewiring to accommodate the splitters. Coincidentally, complexity of the modem is reduced due to the narrower bandwidth and the attendant reduced signal processing bandwidth.
It is desirable to provide dual modality for transmissions wherein a subscriber's line can be used either in the classical ADSL mode as described above, or in a data only mode while providing rapid mode switching between just ADSL service and combined ADSL and POTS service based on central office or subscriber events indicating voice call initiation.